[Edit]

JavaScript - calculate number of pages from size of page and total number of items

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Created by:

Creg

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In this article, we're going to have a look at how in JavaScript, calculate a number of pages when we know:

maximal size of the page,
a total number of items.

Simple example:

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const calculatePagesCount = (pageSize, totalCount) => {
    // we suppose that if we have 0 items we want 1 empty page
    return totalCount < pageSize ? 1 : Math.ceil(totalCount / pageSize);
};
​
​
// Example usage:
​
const pageSize = 10;
const totalCount = 21;
const pagesCount = calculatePagesCount(pageSize, totalCount);
​
console.log('pagesCount=' + pagesCount);  // 3

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The above logic result explanation:

the total number of items is 21 (totalCount variable),
the maximal size of the page is 10 items (pageSize variable),

So:

page 1 should have 10 items
page 2 should have 10 items
page 3 should have 1 item

so we have 3 pages.

Testing

Edit

In this section we present simple test for different calculatePagesCount() usage cases.

xxxxxxxxxx
 
const calculatePagesCount = (pageSize, totalCount) => {
    // we suppose that if we have 0 items we want 1 empty page
    return totalCount < pageSize ? 1 : Math.ceil(totalCount / pageSize);
};
​
​
// Exmaple tests:
​
const pageSize = 10;
​
const testingData = [
//  { input             expected        }
    { totalCount:    0, pagesCount:   1 }, // 1 empty page
    { totalCount:    1, pagesCount:   1 },
    { totalCount:   10, pagesCount:   1 },
    { totalCount:   11, pagesCount:   2 },
    { totalCount:   99, pagesCount:  10 },
    { totalCount:  100, pagesCount:  10 },
    { totalCount:  101, pagesCount:  11 }
];
​
console.log('Pages count:');
console.log('[expected]\t[actual]');
​
for (const entry of testingData) {
    const pagesCount = calculatePagesCount(pageSize, entry.totalCount);
    if (entry.pagesCount !== pagesCount) {
        throw new Error('Pages count calculator error.');
    }
    console.log(entry.pagesCount + '\t\t' + pagesCount); // expected vs actual
}